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Abstract:

A sensor and method for fabricating a sensor is disclosed that in one
embodiment bonds an etched semiconductor substrate wafer to an etched
first device wafer comprising a silicon on insulator wafer which is then
bonded to a second device wafer comprising a silicon on insulator wafer
to create a vented, suspended structure, the flexure of which is sensed
by an embedded sensing element to measure differential pressure. In one
embodiment, interconnect channels embedded in the sensor facilitate
streamlined packaging of the device while accommodating interconnectivity
with other devices.

Claims:

1. A method for fabricating a sensor comprising the steps of: forming a
vent cavity in a substrate wafer extending from the top surface of said
substrate wafer to an exterior surface of said substrate wafer; bonding
said top surface of said substrate wafer to the bottom surface of a first
device wafer, wherein said first device wafer comprises a first device
layer whose bottom surface forms the bottom surface of said first device
wafer, a first handle, and a first insulator layer located between said
first device layer and said first handle; removing said first handle and
said first insulator layer from said first device wafer exposing the top
surface of said first device layer; forming a diaphragm cavity extending
through said first device layer to said vent cavity; bonding the top
surface of said first device layer to the bottom surface of a second
device wafer forming a diaphragm over said diaphragm cavity, wherein said
second device wafer comprises a second device layer whose bottom surface
forms the bottom surface of said second device wafer, a second handle,
and a second insulator layer located between said second device layer and
said second handle; removing said second handle and said second insulator
layer from said second device wafer exposing the top surface of said
second device layer; and placing a sensing element in said second device
layer proximate said diaphragm to sense flexure in said diaphragm.

2. The method for fabricating a sensor of claim 1, further comprising the
step of forming an interconnect channel in a portion of said second
device layer.

3. The method for fabricating a sensor of claim 1, further comprising the
step of forming an interconnect channel in a portion of said first device
layer and said second device layer.

4. The method for fabricating a sensor of claim 1, wherein said substrate
wafer comprises a device layer, an insulator layer, and a handle, wherein
said insulator layer is located between said device layer and said
handle.

5. The method for fabricating a sensor of claim 4, further comprising the
step of removing said handle and said insulator layer from said substrate
wafer.

6. The method for fabricating a sensor of claim 1, wherein said sensor
measures differential pressure.

7. The method for fabricating a sensor of claim 1, wherein said vent
cavity comprises a hollow cavity within said sensor that extends from
said diaphragm cavity through a side of said substrate wafer.

8. The method for fabricating a sensor of claim 1, wherein said vent
cavity comprises a hollow cavity within said sensor that extends from
said diaphragm cavity through both a side and said bottom surface of said
substrate wafer.

9. The method for fabricating a sensor of claim 1, wherein said vent
cavity comprises a hollow cavity within said sensor that extends from
said diaphragm cavity through said bottom surface of said substrate
wafer.

10. The method for fabricating a sensor of claim 1, wherein said step of
forming a vent cavity in a substrate wafer extending from the top surface
of said substrate wafer to an exterior surface of said substrate wafer
comprises the step of thinning said substrate wafer to expose said vent
cavity.

11. The method for fabricating a sensor of claim 10, wherein said vent
cavity comprises a hollow cavity within said sensor that extends from
said diaphragm cavity through both a side and said bottom surface of said
substrate wafer.

12. The method for fabricating a sensor of claim 10, wherein said vent
cavity comprises a hollow cavity within said sensor that extends from
said diaphragm cavity through said bottom surface of said substrate
wafer.

13. A sensor for measuring environmental forces, said sensor comprising:
a vent cavity in a substrate wafer extending from the top surface of said
substrate wafer to an exterior surface of said substrate wafer; a first
device layer, wherein the bottom surface of said first device layer is
bonded to said top surface of said substrate wafer; a diaphragm cavity
extending through said first device layer to said vent cavity; a second
device layer, wherein the bottom surface of said second device layer is
bonded to the top surface of said first device layer to form a diaphragm
over said diaphragm cavity; and a sensing element in said second device
layer proximate said diaphragm to sense flexure in said diaphragm.

14. The sensor of claim 13, further comprising an interconnect channel
extending through a portion of said second device layer.

15. The sensor of claim 13, further comprising an interconnect channel
extending through a portion of said first device layer and through a
portion of said second device layer.

16. The sensor of claim 13, wherein said substrate wafer comprises a
device layer from a silicon on insulator wafer.

17. The sensor of claim 13, wherein said environmental force is
differential pressure.

18. The sensor of claim 13, wherein said vent cavity comprises a hollow
cavity within said sensor that extends from said diaphragm cavity through
a side of said substrate wafer.

19. The sensor of claim 13, wherein said vent cavity comprises a hollow
cavity within said sensor that extends from said diaphragm cavity through
both a side and said bottom surface of said substrate wafer.

20. The sensor of claim 13, wherein said vent cavity comprises a hollow
cavity within said sensor that extends from said diaphragm cavity through
said bottom surface of said substrate wafer.

Description:

BACKGROUND OF THE INVENTION

[0001] The subject matter herein relates generally to semiconductor
microelectromechanical (MEMS) based sensor configurations that can be
used to detect small forces or flexures generated from mechanical stress,
chemo-mechanical stress, thermal stress, electromagnetic fields, and the
like. More particularly, the subject matter disclosed herein relates to a
MEMS based pressure sensor and a method for fabricating the same.

[0002] Advances in semiconductor microelectronic and MEMS based sensors
have served greatly to reduce the size and cost of such sensors. The
electrical and mechanical properties of silicon microsensors have been
well chronicled. Silicon micromachining and semiconductor microelectronic
technologies have blossomed into a vital sensor industry with numerous
practical applications. For instance, micromachined silicon pressure
sensors, acceleration sensors, flow sensors, humidity sensors,
microphones, mechanical oscillators, optical and RF switches and
attenuators, microvalves, ink jet print heads, atomic force microscopy
tips and the like are widely known to have found their way into various
applications in high volume medical, aerospace, industrial and automotive
markets. The high strength, elasticity, and resilience of silicon makes
it an ideal base material for resonant structures that may, for example,
be useful for electronic frequency control or sensor structures. Even
consumer items such as watches, scuba diving equipment and hand-held tire
pressure gauges may incorporate silicon micromachined sensors.

[0003] The demand for silicon sensors in ever expanding fields of use
continues to fuel a need for new and different silicon microsensor
geometries and configurations optimized for particular environments and
applications. Unfortunately, a drawback of traditional bulk silicon
micromachining techniques has been that the contours and geometries of
the resulting silicon microstructures have been significantly limited by
the fabrication methods. For instance, etching silicon structures with
conventional etching techniques is constrained, in part, by the crystal
orientations of silicon substrates, which limits the geometry and
miniaturization efforts of many desired structures.

[0004] The increasing use of microsensors to measure pressure has spurred
the development of small silicon plate structures used, for example, as
capacitors and to produce electrostatic forces. For instance, there exist
microsensors that measure capacitance using an array of interdigitated
polysilicon plates. Similarly, there exist microsensors that produce
electrostatic forces using an array of layered plates. Further, there
exist microsensors that measure the flexure, or bending, of silicon
structures in response to forces such as pressure or acceleration.

[0005] Measurements of biological parameters using microsensors are
becoming increasingly common and important for both diagnostic and
patient monitoring purposes. In some applications, in-vivo catheter tip
pressure sensors are used to measure either absolute pressure or
differential pressure based on a given reference pressure, such as
atmospheric pressure. For example, differential catheter tip pressure
sensors can be used to measure the breathing of a human being based on
pressure changes within the respiratory system with respect to
atmospheric pressure. The expanding fields of use of
microelectromechanical devices in general, and of catheter tip pressure
sensors in particular, has created a demand for ever smaller devices.
Unfortunately, there has been difficulty producing smaller devices that
are also highly sensitive to small changes in pressure which can be
effectively manufactured in high volumes.

[0006] Sensors manufactured through conventional fabrication techniques
are limited with respect to their size and packaging. For example, the
elongated nature of a catheter tip pressure sensor requires that
electrical connections extend from one end of the sensor, typically the
end that is not inserted, to the sensing portion of the device. These
connections can detrimentally impact the size and shape of the resulting
device. Additionally, because of the small size of the devices and the
thin nature of the geometries used, conventional techniques for producing
such micromechanical devices risk both breakage during the manufacturing
process and potentially diminished reliability in the field. For example,
since differential catheter tip pressure sensors measure pressure
relative to a reference pressure, a vent from the sensor to an external
reference pressure must be supplied. This is typically done through a
fine capillary tube that is run to the catheter tip in parallel with the
electrical connections along the back of the chip. However, this
configuration can result in thicker packaging of the sensor and can
result in the vent becoming pinched-off during measurement. Other
fabrication techniques employ side vent configurations that exit the chip
through vent ports located on one of the chip edges, but which require
additional processing steps to create the vent port, such as sawing, that
can result in entry of debris into the vent port and diminish both
accuracy and reliability.

[0007] It would be advantageous to provide a method for manufacturing
highly sensitive pressure sensors that are not only small in size, but
which can be effectively produced in high volume.

BRIEF DESCRIPTION OF THE INVENTION

[0008] A sensor and a method for fabricating a sensor is disclosed, in one
embodiment comprising a vent cavity in a substrate wafer extending from
the top surface of the substrate wafer to an exterior surface of the
substrate wafer, a first device layer, wherein the bottom surface of the
first device layer is bonded to the top surface of said substrate wafer,
a diaphragm cavity extending through the first device layer to the vent
cavity, a second device layer, wherein the bottom surface of the second
device layer is bonded to the top surface of the first device layer to
form a diaphragm over the diaphragm cavity, and a sensing element in the
second device layer proximate the diaphragm to sense flexure in the
diaphragm.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] So that the manner in which the features of the invention can be
understood, a detailed description of the invention may be had by
reference to certain embodiments, some of which are illustrated in the
accompanying drawings. It is to be noted, however, that the drawings
illustrate only certain embodiments of this invention and are therefore
not to be considered limiting of its scope, for the scope of the
invention encompasses other equally effective embodiments. The drawings
are not necessarily to scale, emphasis generally being placed upon
illustrating the features of certain embodiments of invention. Thus, for
further understanding of the invention, reference can be made to the
following detailed description, read in connection with the drawings in
which:

[0010]FIG. 1 is a cross sectional view of an exemplary differential
pressure sensor in one embodiment of the invention.

[0011]FIG. 2 is a cross sectional view of two exemplary silicon on
insulator device wafers and an exemplary substrate wafer used to
fabricate a differential pressure sensor in one embodiment of the
invention.

[0012]FIG. 3 is an exemplary top view of a differential pressure sensor
illustrating an embedded vent in one embodiment of the invention.

[0013]FIG. 4 is an exemplary substrate wafer with vent bonded to a first
device layer with diaphragm cavity bonded to a second device wafer in one
embodiment of the invention.

[0014]FIG. 5 is an exemplary substrate wafer with etched side vent in one
embodiment of the invention.

[0015]FIG. 6 is an exemplary substrate wafer with etched side and bottom
vent in one embodiment of the invention.

[0016]FIG. 7 is an exemplary substrate wafer with etched bottom vent in
one embodiment of the invention.

[0017]FIG. 8 is an exemplary substrate wafer with vent bonded to a first
device layer with diaphragm cavity bonded to a second device layer having
an interconnect channel, sensing element and interconnect in one
embodiment of the invention.

[0018]FIG. 9 is an exemplary process flow for fabricating a differential
pressure sensor in one embodiment of the invention.

[0019]FIG. 10 is an exemplary cross sectional view of a proximal side of
a differential pressure sensor in one embodiment of the invention.

[0020]FIG. 11 is an exemplary absolute pressure sensor in one embodiment
of the invention.

[0021]FIG. 12 is an exemplary process flow for fabricating an absolute
pressure sensor in one embodiment of the invention.

[0022]FIG. 13 is an exemplary cross sectional view of a proximal side of
an absolute pressure sensor in one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0023] An exemplary micromachined pressure sensor can be made by forming a
cavity within a silicon structure and a diaphragm adjacent to the cavity.
In differential pressure sensor embodiments, the cavity is connected to a
vent that provides access to the cavity from outside the sensor, either
through the side, bottom, or combination side and bottom of the sensor.
For an absolute pressure sensor, in which measurements are made in
relation to a selected reference pressure, the cavity can be held in a
vacuum or a selected internal pressure. The pressure sensor measures
pressure by sensing the flexure of the diaphragm, for example how the
pressure acting on the front side of the diaphragm deflects the diaphragm
inwards. One or more sensing elements formed near the edges of the
diaphragm typically sense the flexure or deflection of the diaphragm.

[0024]FIG. 1 is an exemplary cross sectional view of a differential
pressure sensor 10 having a bottom vent cavity 330 in one embodiment of
the invention. Pressure sensor 10 can have a distal end 50 containing the
pressure sensing elements of the device that, in one embodiment, comprise
the tip of the catheter that can be insertable into a medium, for example
a patient's respiratory system, for taking pressure measurements.
Pressure sensor 10 can also have a proximal end 75 that can be capable of
electrically coupling with other devices for reading and processing the
pressure measurements.

[0025] Pressure sensor 10 can be manufactured using three wafers that are
processed and bonded together, for example two silicon on insulator (SOI)
semiconductor wafers and a double side polished (DSP) semiconductor
wafer, or three SOI wafers. FIG. 2 shows three exemplary starting wafers
in one embodiment of the invention. Device wafers 100 and 200 can be SOI
wafers having device layers 110 and 210, insulator layers 115 and 215,
and handle layers 120 and 220, respectively. Device layer 110 can be a
single crystal silicon substrate which, in one embodiment, can be 1 to 10
μm thick and have n-type doping. Device layer 210 can be a single
crystal silicon substrate which, in one embodiment, can be a thickness
selected to meet particular design specifications, and which can have
n-type or p-type doping. The thickness of the various layers of the SOI
wafer can be precisely set using conventional SOI chip manufacturing
techniques, and can be selected such that the precise thickness of the
layers determine the subsequent operating characteristics of the pressure
sensor 10, as will be described below. Insulator layers 115 and 215 can,
in one embodiment, be silicon dioxide and range between 0.05 μm to 1.0
μm thick. Handle layers 120 and 220 can be used to grip the device
wafers 100 and 200, respectively, during the manufacturing process, and
can be located such that the insulator layers 115 and 215 are positioned
between the device layers 110 and 210 and the handle layers 120 and 220,
respectively. Handle layers 120 and 220 can consist of, for example,
n-type or p-type silicon having a thickness between 200 μm to 600
μm. In one embodiment, substrate wafer 300 can be a double side
polished silicon wafer which, in one embodiment, can be 300 μm to 600
μm thick and have n-type or p-type doping. In other embodiments,
substrate wafer 300 can be a third SOI wafer. Together, the thicknesses
of the various layers comprising the pressure sensor 10 can be selected
such that the overall thickness of the device in one embodiment can be
390 μm or less.

[0026] With reference again to FIG. 1, pressure sensor 10 can be comprised
of device layer 110, device layer 210 and substrate wafer 300. One or
more sensing elements 140, for example p-type piezoresistive sensing
elements, can be strategically implanted or diffused within the device
layer 110 to sense flexure in the silicon structures. Pressure sensor 10
can also include passivation layers 170 and 470 that can consist of, for
example, a silicon dioxide layer, a silicon nitride layer, or a
combination of both. Passivation layers 170 and 470 can provide
insulation and protection to pressure sensor 10 during manufacturing and
operation. One or more interconnects 150 formed on device layer 110 can
electrically couple one or more sensing elements 140 to the exterior of
the pressure sensor 10, while one or more metallization layers 160 can
provide electrical connectivity between the interconnects 150 and the
proximal end 75 of the pressure sensor 10 such that the pressure sensor
can be electrically coupled to other devices or connections through, for
example, a lead attachment.

[0027] With reference to FIGS. 1 and 3, an exemplary differential pressure
sensor 10 and operation thereof is described in one embodiment of the
invention. FIG. 3 is an exemplary top view of a differential pressure
sensor 10 illustrating an embedded vent cavity 330 in one embodiment of
the invention. Dotted lines in FIG. 3 depict exemplary embedded
structures within the pressure sensor 10 that form the vent 330, while
dashed lines in FIG. 3 depict an exemplary diaphragm cavity 230 embedded
within pressure sensor 10. Pressure sensor 10 operates by measuring
flexure in a thinned structure or diaphragm 130 formed in device layer
110 over a diaphragm cavity 230 formed in device layer 210, which is
bonded between substrate wafer 300 and device wafer 110. The diaphragm
serves as a flexure structure in pressure sensor 10. Vent cavity 330
connects diaphragm cavity 230 to the exterior of pressure sensor 10
through a hollow vent channel 333 beginning at a vent recess 332
connected to the diaphragm cavity 230 and extending through the substrate
wafer 300 to a vent outlet 335 that opens to the exterior. As the
pressure above the diaphragm 130 changes, the diaphragm 130 will flex
towards or away from the diaphragm cavity 230 in relation to the pressure
at the vent outlet 335. The elongated structure of the pressure sensor 10
can allow the pressure sensor 10 to operate as a catheter tip pressure
sensor such that the portion of the pressure sensor 10 comprising the
diaphragm 130 can be inserted into a medium, for example a patient's
respiratory system or bloodstream, while the vent outlet 335 remains
exposed to an exterior pressure gradient, for example atmospheric
pressure, thereby providing differential pressure measurement.

[0028] Diaphragm 130 will flex in relation to the diaphragm cavity 230 in
a predictable way from pressure exerted on the diaphragm 130. The flexure
in diaphragm 130 can be detected by one or more sensing elements 140
formed in device layer 110 on or near the edges of diaphragm 130. In one
embodiment using piezoresistive sensing elements, the resistance of
sensing element 140 can be determined via a circuit, such as a wheatstone
bridge circuit or the like, interconnected using one or more
interconnects 150 attached to one or more metallization layers 160 that
can extend from the interconnects 150 through interconnect channels 400
formed in device layer 110, or both device layers 110 and 210, to the
proximal end 75 of the pressure sensor 10. An electrical interface or
other such device can be attached to the ends of the metallization layers
160 to place the pressure sensor 10 in electrical communication with
another device. The resistance of the piezoresistive sensing element
varies with the flexure of diaphragm 130. Thus, measurement of the
piezoresistive resistance of sensing element 140 can be used to determine
the amount of flexure in diaphragm 130, and thereby determine the
pressure exerted on the sensor.

[0029] An exemplary process for fabricating a silicon sensor like the one
illustrated in FIG. 1 is explained with reference to FIGS. 1 through 10.
FIG. 9 is an exemplary process flow for fabricating a differential
pressure sensor 10 in one embodiment of the invention. In step 501 of
FIG. 9, vent recess 332, vent channel 333 and vent outlet 335, which
together can form the vent cavity 330 in pressure sensor 10, can be
formed on an upper substrate surface 310 of substrate wafer 300 using
standard semiconductor etch techniques such as dry etching using deep
reactive ion etching (DRIE), wet etching with potassium hydroxide (KOH)
or tetramethylammonium hydroxide (TMAH), or other silicon etchants or the
like. In one embodiment, a first etch is made to form the vent recess 332
and vent channel 333, followed by a second etch to form the vent outlet
335.

[0030] As shown in FIGS. 5 through 7, different etch geometries can be
employed to form recesses on the upper substrate surface 310 of substrate
wafer 300 to achieve different vent cavity configurations. In one
embodiment, shown in FIG. 5, vent cavity 330 can be a side vent
configuration in which vent outlet 335 forms an opening on a side of the
substrate wafer 330. In other embodiments having a bottom or a
combination side/bottom vent cavity 330 configuration, later fabrication
steps can include thinning the substrate wafer 300 to expose the vent
outlet 335 to the exterior of the differential pressure sensor 10. For
example, in bottom vent and side/bottom vent embodiments, the bottom
substrate surface 320 of substrate wafer 300 can be removed up to the
thickness indicator 350 shown in FIGS. 6 and 7. In the embodiment shown
in FIG. 6, when the bottom substrate surface 320 of substrate wafer 300
is thinned to the thickness indicator 350 a side/bottom vent cavity 330
with opening on both the side and bottom of substrate wafer 330 is
formed. In another embodiment, shown in FIG. 7, when the bottom substrate
surface 320 of substrate wafer 300 is thinned to the thickness indicator
350 a bottom vent with an opening on the bottom substrate surface 320 of
substrate wafer 300 is formed.

[0031]FIG. 4 is an exemplary substrate wafer 300 with vent cavity 330
bonded to a first device layer 210 with diaphragm cavity 230 bonded to a
second device wafer 100 in one embodiment of the invention. With
reference to FIGS. 4 and 9, step 502 in the fabrication process can be to
bond the device layer 210 of device wafer 200 to the upper substrate
surface 310 of the substrate wafer 300 using conventional silicon fusion
bonding techniques. In one exemplary fusion bonding technique, the
opposing surfaces can be made hydrophilic. That is, the surfaces can be
treated with a strong oxidizing agent that causes water to adhere to
them. The two wafers can then be placed in a high temperature environment
for a period of time demanded by the quality of the bond. This silicon
fusion bonding technique bonds the substrate wafer 300 and the device
wafer 200 together without the use of an intermediate adhesive material
that could have a different coefficient of thermal expansion than the
single crystal silicon wafer. Fusion bonding can also be performed in
which oxide layers are formed on the bonded surfaces of one or both of
the wafers.

[0032] In step 503, after the upper substrate surface 310 of the substrate
wafer 300 and device layer 210 have been bonded, the handle layer 220 of
the device wafer 200 can be removed using a wet etchant, such as KOH or
TMAH, that stops on the insulator layer 215. Additionally, insulator
layer 215 can be removed using wet or dry etching techniques, leaving
only the bonded device layer 210, which is now exposed.

[0033] In step 504, the diaphragm cavity 230, which can be a hole
extending through the device layer 210, can be etched into device layer
210 using DRIE, wet etching with KOH or TMAH, or other silicon etchants
or the like. Diaphragm cavity 230 can have various geometries, for
example square, rectangle or circular, and can have any required depth,
for example, from less than 5 microns to greater than 100 microns,
depending on the particular application and the chosen thickness of
device layer 210. The surfaces of diaphragm cavity 230 and vent cavity
330 can be either bare silicon, oxidized silicon, doped silicon, or they
can be coated with any other thin film capable of withstanding subsequent
wafer bonding and processing temperatures.

[0034] In step 505, device layer 110 of device wafer 100 can be bonded to
the device layer 210 of device wafer 200 using conventional silicon
fusion bonding techniques to form a device pair 450. In one exemplary
fusion bonding technique, the opposing surfaces can be made hydrophilic.
That is, the surfaces can be treated with a strong oxidizing agent that
causes water to adhere to them. The two wafers can then be placed in a
high temperature environment for a period of time demanded by the quality
of the bond. This silicon fusion bonding technique bonds device wafer 100
and device wafer 200 together without the use of an intermediate adhesive
material that could have a different coefficient of thermal expansion
than the single crystal silicon wafer. Fusion bonding can also be
performed in which oxide layers are formed in the bonded surfaces of one
or both of the wafers.

[0035] In step 506, after the opposing surfaces of the device layer 210
and device 110 have been bonded, the handle layer 120 of the device wafer
100 can be removed using a wet etchant, such as KOH or TMAH, that stops
on the insulator layer 115. Additionally, insulator layer 115 can be
removed using wet or dry etching techniques, leaving the non-bonded
device layer 110 exposed.

[0036]FIG. 8 is an exemplary substrate wafer 300 with vent cavity 330
bonded to a first device layer 210 with diaphragm cavity 230 bonded to a
second device layer 110 having an interconnection channel 400, sensing
element 140 and interconnect 150 in one embodiment of the invention. With
reference to FIGS. 8 and 9, in step 507, passivation layer 170 can be
deposited on the non-bonded surface of device layer 110 using, for
example, a silicon dioxide layer, a silicon nitride layer, or
combinations of both to properly insulate and protect the device wafer
110 during both the manufacturing process and operation. In step 508, one
or more sensing elements 140 can be added by diffusion or ion implanting
of, in one preferred embodiment using piezoresistive sensing elements,
low doped p-type material into the doped n-type device layer 110 near the
edges of diaphragm 130, which can be formed as part of the device layer
110. For example, Boron implantation and diffusion at high temperature
may form one or more piezoresistive sensing elements within device layer
110. The piezoresistive sensing elements can be positioned to sense
flexure in diaphragm 130. It should be noted that any number of
piezoresistive sensing elements may be employed and their exact
positioning relative to the diaphragm 130 may be different depending on
the particular application, expected pressures, sensitivity requirements,
and the like. Additionally, one or more interconnects 150 that can
provide electrical conductivity to the sensing elements 140 can be added
by diffusion or ion implanting of highly doped p-type material into the
doped n-type device layer 110 overlapping sensing elements 140. Diffusion
or implantation of sensing element 140 and interconnect 150 can be done
in individual steps or in a single step.

[0037] In step 509, one or more interconnect channels 400 that provide a
passageway along the pressure sensor 10 for conductors can be etched, in
one embodiment, into device layers 110 and 210. As shown in FIG. 8,
passivation layer 170 can be etched using dry or wet etching techniques
to define the location of the interconnect channels 400 on the proximal
end 75 of the pressure sensor 10. Once the passivation layer 170 has been
removed, one or more interconnect channels 400 can be etched into device
layer 110, and optionally device layer 210, through wet etching
techniques, for example using KOH or TMAH. In one embodiment,
interconnect channels 400 are etched to form a series of spaced apart
v-shaped grooves, as shown in an exemplary cross section of the proximal
end 75 of the pressure sensor 10 shown in FIG. 10. Interconnect channels
400 can facilitate the later attachment of one or more conductors to the
proximal end 75 of pressure sensor 10 while maintaining the small,
elongated packaging required for some applications, such as catheter tip
pressure sensors.

[0038] With reference to FIGS. 9 and 10, in step 510 passivation layer 470
can be deposited on the surface of interconnect channels 400 using, for
example, a silicon dioxide layer, a silicon nitride layer, or a
combination of both to properly insulate and protect the interconnect
channels 400 during both the manufacturing process and operation.
Additionally, passivation layer 170 can be etched using dry or wet
etching techniques to access interconnect 150. In step 511, metallization
layer 160 can be added, providing electrical conductivity from the
proximal end 75 of pressure sensor 10 to the sensing elements 140 through
interconnects 150. Metallization layer 160 can be formed of, for example,
gold or aluminum, and can be created to a desired thickness to suit
device design needs.

[0039] In embodiments, in which substrate wafer 300 is an SOI wafer, the
handle layer of the substrate wafer 300 can be removed using a wet
etchant, such as KOH or TMAH, that stops on the insulator layer.
Additionally, the insulator layer can be removed using wet or dry etching
techniques, leaving only the device layer of the SOI wafer containing the
vent 300 bonded to the device layer 210.

[0040] Lastly, if the pressure sensor 10 is a bottom or side/bottom vent
embodiment, in step 512 the substrate wafer 300 can be thinned down using
dry etching or wet etching techniques, for example using KOH or TMAH, to
expose the vent outlet 335 on the exterior of the pressure sensor 10.

[0041]FIG. 11 shows one exemplary embodiment in which pressure sensor 10
can be fabricated such that a vacuum, or other selected pressure, is
formed within the diaphragm cavity 230 so that pressure sensor 10 is an
absolute pressure sensor. In this embodiment, pressure measurements are
not taken in relation to the exterior pressure of the sensor, so that
fabrication of a vent within the sensor is unnecessary. FIG. 12 is an
exemplary process flow showing the steps that can be performed to
fabricate an absolute pressure sensor 10 in one embodiment of the
invention. The techniques employed are similar to those in fabricating a
differential pressure sensor, but because this embodiment does not
require a vent, the pressure sensor 10 can be fabricated using only two
wafers and, accordingly, some fabrications steps can be eliminated. In
one embodiment, the absolute pressure sensor 10 can comprise an SOI
device wafer 100 and a DSP substrate wafer 300. In another embodiment,
absolute pressure sensor 10 can comprise a first SOI device wafer 100 and
the substrate wafer 300 can comprise a second SOI device wafer.

[0042] With reference to FIGS. 11 and 12, in step 601, the diaphragm
cavity 230 can be etched directly into the substrate wafer 300 using
DRIE, wet etching with KOH or TMAH, or other silicon etchants or the
like. Diaphragm cavity 230 can have various geometries, for example
square, rectangular or circular, and can have any required depth, for
example, from less than 5 microns to greater than 100 microns, depending
on the particular application and the chosen thickness of the substrate
wafer 300. The surface of diaphragm cavity 230 can be either bare
silicon, oxidized silicon, doped silicon, or it can be coated with any
other thin film capable of withstanding subsequent wafer bonding and
processing temperatures.

[0043] In step 602, the device layer 110 of device wafer 100 is bonded to
the surface of the substrate wafer 300 using conventional silicon fusion
bonding techniques to form a device pair 450. In one exemplary fusion
bonding technique, the opposing surfaces can be made hydrophilic. That
is, the surfaces can be treated with a strong oxidizing agent that causes
water to adhere to them. The two wafers can then be placed in a high
temperature environment for a period of time demanded by the quality of
the bond. The silicon fusion bonding technique described above bonds the
substrate wafer 300 and the device wafer 100 together without the use of
an intermediate adhesive material that could have a different coefficient
of thermal expansion than the single crystal silicon wafer. Fusion
bonding can also be performed in which oxide layers are formed on the
bonded surfaces of one or both of the wafers.

[0044] In step 603, after the opposing surfaces of the substrate wafer 300
and device layer 110 have been bonded, the handle layer 120 of the device
wafer 100 can be removed using a wet etchant, such as KOH or TMAH, that
stops on the insulator layer 115. Additionally, insulator layer 115 can
be removed using wet or dry etching techniques, leaving only the bonded
device layer 110, whose non-bonded top surface is now exposed.
Additionally, in step 604 passivation layer 170 can be deposited on the
non-bonded top surface of device layer 110 using, for example, silicon
dioxide, silicon nitride layers, or combinations of both to properly
insulate and protect the device wafer 110 during both the manufacturing
process and operation.

[0045] With reference to step 605 of FIG. 12, one or more sensing elements
140 can be added by diffusion or ion implanting of, in one preferred
embodiment using piezoresistive sensing elements, low doped p-type
material into the doped n-type device layer 110 near the edges of
diaphragm 130, which can be formed as part of the device layer 110. For
example, Boron implantation and diffusion at high temperature may form
piezoresistive sensing elements 140 within device layer 110. The sensing
elements 140 can be positioned to sense flexure in diaphragm 130. It
should be noted that any number of sensing elements 140 may be employed
and their exact positioning relative to the diaphragm 130 may be
different depending on the particular application, expected pressures,
sensitivity requirements, and the like. Additionally, one or more
interconnects 150 that can provide electrical conductivity to the sensing
elements 140 can be added by diffusion or ion implanting of highly doped
p-type material into the doped n-type device layer 110 overlapping to
sensing elements 140.

[0046] In step 606, one or more interconnect channels 400 that provide a
passageway along the pressure sensor 10 for various conductors can be
etched, in one embodiment, into device layers 110 and 210. First, as
shown in FIG. 11, passivation layer 170 is etched using dry or wet
etching techniques to define the location of the interconnect channels
400 on the proximal end of the pressure sensor opposite the end
comprising the diaphragm. Once the passivation layer 170 has been
removed, one or more interconnect channels 400 can be etched into device
layer 110, and optionally device layer 210, through wet etching
techniques, for example using KOH or TMAH. In one embodiment,
interconnect channels 400 are etched to form a series of spaced apart
v-shaped grooves, as shown in an exemplary cross section of the proximal
end 75 of the pressure sensor 10 shown in FIG. 13. Interconnect channels
400 allow for the later connection of one or more conductors to the
proximal end 75 of pressure sensor 10 while maintaining the small,
elongated packaging required for some applications, such as catheter tip
pressure sensors.

[0047] With reference again to FIGS. 11 and 12, in step 607 passivation
layer 470 can be deposited on the surface of interconnect channels 400
using, for example, a silicon dioxide layer, a silicon nitride layer, or
combinations of both to properly insulate and protect the interconnect
channels 400 during both the manufacturing process and operation.
Additionally, passivation layer 170 can be etched using dry or wet
etching techniques to expose the interconnect 150. In step 608,
metallization layer 160 can be added, providing electrical conductivity
from the proximal end 75 of pressure sensor 10 to the sensing element 140
through interconnect 150. Metallization layer 160 can be formed of, for
example, gold or aluminum, and can be created to a desired thickness to
suit device design needs.

[0048] Substrate wafer 300 can be thinned using conventional etching
techniques to accommodate given design specifications and thickness
requirements for pressure sensor 10. In addition, in embodiments, in
which substrate wafer 300 is an SOI wafer, the handle layer of the
substrate wafer 300 can be removed using a wet etchant, such as KOH or
TMAH, that stops on the insulator layer. Additionally, the insulator
layer can be removed using wet or dry etching techniques, leaving only
the device layer of the SOI wafer containing the vent 300 bonded to the
device layer 110.

[0049] With reference to both differential and absolute pressure sensor
embodiments described herein, each etch made during the fabrication of
pressure sensor 10 can have any chosen geometry and can have any required
depth depending on the particular application. Additionally, the etches
need not have a single, uniform depth, and the resulting etches can be
isotropic or anisotropic. The selected depth and geometry of each etch
can be selected to alter the design characteristics of the resulting
pressure sensor 10. For example, the thickness of device layer 110 and
the size and shape of the diaphragm 130 dictated by diaphragm cavity 230
can be selected to determine the sensitivity of the resulting pressure
sensor 10. The selected thickness of device layer 110, which can be
arbitrarily chosen and precisely controlled in manufacturing the SOI
wafer, leads to improved control over the flexibility of diaphragm 130,
and therefore improved control over the performance characteristics of
the resulting pressure sensor 10. Additionally, the planar manufacturing
processes are ideal for manufacturing purposes and can increase not only
the fabrication yield, but the overall reliability and long term
performance of the resulting devices. Accordingly, uniform control over
the performance characteristics of the pressure sensor 10 can be
achieved.

[0050] The above detailed description is provided to illustrate exemplary
embodiments and is not intended to be limiting. Although the method for
fabricating a sensor has been shown and described with respect to
embodiments which measure pressure, it will be apparent to those skilled
in the art that similar techniques can be used to fabricate sensors
capable of measuring other parameters. For example, it should be
recognized that although various exemplary embodiments of the sensor and
associated methods of manufacture disclosed herein have been described
with reference to various catheter tip medical applications, the
apparatus and method of manufacture are useful in a wide variety of other
applications not explicitly described herein. It will also be apparent to
those skilled in the art that numerous modifications and variations
within the scope of the present invention are possible. Further, numerous
other materials and processes can be used within the scope of the
exemplary methods and structures described as will be recognized by those
skilled in the art. For example, it should be recognized that the p-type
and n-type materials described herein can be used in an alternative
fashion, e.g., by replacing p-type materials for n-type materials and
vice versa. Additionally, it will be apparent to those of skill in the
art that the sequence of steps identified and described in various
exemplary embodiments need not occur in the sequence described, and that
in other embodiments various steps can be combined, performed in
different orders, or in parallel and still achieve the same result.

[0051] This written description uses examples to disclose the invention,
including the best mode, and also to enable any person skilled in the art
to practice the invention, including making and using any devices or
systems and performing any incorporated methods. The patentable scope of
the invention is defined by the claims, and may include other examples
that occur to those skilled in the art. Such other examples are intended
to be within the scope of the claims if they have structural elements
that do not differ from the literal language of the claims, or if they
include equivalent structural elements with insubstantial differences
from the literal language of the claims.